July 26, 1908
The FBI is born
Until 1908, the Department of Justice used the Secret Service to conduct federal investigations when they were needed, but the operatives reported to the SS instead of the DOJ. Finally, in early 1908, congress passed a law which stopped the DOJ from engaging SS operatives.
On July 26, 1908, Attorney General Bonaparte appointed special agents and ordered them to report directly to Chief Examiner Stanley W. Finch--an action which is marked as the beginning of the Federal Bureau of Investigations.
http://www.fbi.gov/about-us/history/brief-history